Conductive adhesives, comprising conductive particles in an adhesive base, are well known. They have many applications but are particularly useful in the manufacture of electronic devices to provide adhesion and electrical connection between components such as in liquid crystal displays (LCDs), LCD screens and driver electronics.
Anisotropic conductive adhesives (ACA) and anisotropic conductive films (ACF) that pass electricity along only one axis provide electrical connection in many critical electronic systems. This approach can replace traditional methods, like soldering, and can provide connectivity where conventional technologies often fail. ACA/ACF also facilitate a more efficient use of the board ‘real estate’ as well as more flexible and reliable interconnects. Typical ACA pastes contain electrically conductive metallic particles (typically metal-coated polymer particles), ranging in size from 2 to 50 micrometers, incorporated in an insulating binder. Larger particles are used in applications such as “Flip Chip”, where an unprotected device is mounted “face down” onto the interconnect board.
ACA/ACF is widely used in the electronics industry, and has become the de-facto standard for the interconnect of driver electronics to displays for LCD manufacturing. The ACA/ACF is applied to a substrate, and the component is then placed accurately on the substrate so that the contacts on the component and substrate align. A force is applied and at the same time the curing process of the adhesive is activated. This could be by any of a number of methods, including contact heating, infra-red heating, microwave heating or UV light. During this process, conductive particles are trapped between the mating contact bumps. To minimise the amount of voids in the final adhesive connection, an excess of adhesive is used which must be squeezed out during the bonding process. Due to the strongly time and temperature dependent properties of the adhesive caused by the on-going curing, it is immensely difficult to predict the right combination of force, time and temperature that will correctly squeeze out the right amount of adhesive and leave the particles with the correct amount of deformation after the finished bonding. These parameters have to be adjusted by experiments and in some cases even monitored continuously during manufacturing, but it is still very difficult to get this right. Thus, determining the correct process parameters that provide the correct amount of deformation of the contact particles is a problem.
Over the years, the LCD industry has developed a technique based on optical inspection of the bonded particles. As the particles are to some extent brittle, they will crack when the deformation reaches certain levels. The particles have a significant variation in size and mechanical properties (due to lack of homogeneity in the manufacturing process), and due to this variation, there will be a wide variation in the diameter at which the particles fail when they are deformed (termed here the “crack point”). Because of this wide distribution of crack points a population of particles will gradually crack as pressure is applied and the particles are deformed (e.g. compressed). The technique assumes that the desired amount of deformation has been achieved when a certain (small) percentage of the particles have cracked. The remaining uncracked particles then provide conductivity. The process parameters (e.g. pressure and temperature) at this point are then taken to be the correct parameters to apply.
In practice, components are applied to the substrate, cured and the fraction of particles between the contacts that are cracked or crushed is determined. A nominal fraction, for example 10 to 20%, is used as an indication that a desired deformation has been achieved, e.g. sufficient pressure has been applied. Less than this fraction and the pressure was too low to give optimum contact. More than this and the pressure was too high. Thus, by inspecting the fraction of particles that are cracked or crushed, the bonding process window can be estimated. In cases where the substrate is transparent, such as in the case of LCD manufacturing, it is straightforward to inspect the cracked particles. In other cases it is more difficult.
The present inventors have however recognised a problem with this technique. As electronic devices become smaller and smaller, the size of and pitch between contacts also becomes smaller and the conductive particle size used in the conductive adhesive becomes smaller. As LCD technology develops, the display resolution and pixel count continues to increase, consequently the pitch between contacts becomes smaller and thus the polymer-core sizes of the conductive particles have progressively reduced. At the same time, the cost of the driver ICs is strongly dependent on the silicon area of the IC, which today is defined by the pad size and the number of pads. Using smaller particles will allow smaller pads (or more contacts per pad), and will therefore significantly reduce the cost of the driver ICs.
For the smallest particle sizes in particular, there are significant advantages in having extremely small size distributions of the particles. Whereas previously, a coefficient of variation (CV) of 10% was adequate, CVs of <5%, preferably <3% or even <2% are now desired.CV is defined as: CV=100×standard deviation of diameter/average diameterAt the same time, a better control and homogeneity of the mechanical properties of the particles are needed, to fulfil the reliability requirements of the new generation of ACF materials.
In LCD technology, the LCD glass is flat on a sub-micron level. The main planarity issue has typically been with the contact-bumps on the chips themselves. However, it is now becoming feasible to planarize the bumps by mechanical means (lapping) on the wafer level, with the resultant situation being that all pad substrate distances become very uniform. Hence, particle homogeneity becomes the critical aspect.
The very small distribution of size and mechanical properties in the latest generation of particles means that there is also a very small distribution of crack points. In other words, all the particles tend to crack when they are deformed (compressed) to an almost identical size. The applicant, Conpart AS, manufacture particles with an extreme homogeneity of crackpoints, as documented in the following paper: He J Y, Zhang Z L, Kristiansen H. Int J Mater Res 2007; 98:389-92. Therefore even if such particles are deformed very slowly (e.g. pressure is slowly applied), the particles will all tend to crack at the same point. This makes it practically impossible to allow just a small percentage to crack in order to estimate when desired deformation has been achieved whilst allowing the majority of particles to remain intact. In other words, the lack of particle-to-particle variation combined with the uniformity of substrate and pad-planarity makes it near impossible to establish a usable process window based on fracture of the particles.
Thus, the industry's need for smaller particles with better and more homogenous performance, and hence a very small distribution of stress and strain at failure, is not compatible with the current methods for determining a reasonable process window. This problem will hamper the development of ACF technology in future because it will become more and more difficult to achieve the correct amount of deformation of the particles.
Moreover, the current method can only provide an approximate indication of the bonding process window, it does not clearly identify the point at which the desired deformation has been achieved (which will vary from batch to batch of particles). The present inventors have devised a new process for determining the optimum bonding pressure in order to secure two substrates together using conductive adhesives even using particles with low coefficient of variation. The technique relies on the use of a small population of signal particles in combination with the conductive adhesive particles, the signal particles indicating when an optimum bonding pressure has been applied before the conductive particles themselves are damaged, e.g. through cracking